1. Field of the Invention
The invention relates to gas pressure regulation in vapor deposition, and more particularly to a vapor deposition apparatus and a method of regulating a gas pressure in gas feed lines in a vapor deposition apparatus, both suitable for the formation of a multilayer crystal or a highly purified crystal layer having a uniform, steep structural interface, on a semiconductor substrate.
2. Description of the Related Art
When a multilayer crystal film or a highly purified crystal layer is formed on a semiconductor substrate by means of a conventional vapor deposition apparatus, performances of a semiconductor device are significantly influenced by evenness of a crystal interface and/or abrupt changes in gases fed for the formation of a semiconductor device. Thus, various attempts have been made in order to enhance the evenness of a crystal interface and smooth the abrupt changes in gases. Hereinbelow will be explained one of conventional vapor deposition apparatuses.
FIG. 1 illustrates a pipe arrangement upstream of a reaction pipe in a conventional vapor deposition apparatus. As illustrated in FIG. 1, the vapor deposition apparatus includes a first gas source 1 for providing a first gas, a second gas source 2 for providing a second gas, a compensation gas source 3 for supplying a gas to compensate for flow rates in a later mentioned reaction pipe line 4 and a vent line 5, a reactive pipe line 4 in which a gas actually used for vapor deposition is mixed with a principal carrier gas and through which the mixture gas is introduced into a reaction pipe (not illustrated), and a vent line 5 through which a gas not used for vapor deposition is introduced into an exhauster (not illustrated ) together with a secondary carrier gas. The first gas source 1 is in fluid communication with the reactive pipe line 4 and the vent line 5 at inlet ports 1a and 1b through valves 10 and 11, respectively. Similarly, the second gas source 2 is in fluid communication with the reactive pipe line 4 and vent line 5 at inlet ports 2a and 2b through valves 12 and 13, respectively, and the compensation gas source 3 is in fluid communication with the reactive pipe line 4 and vent line 5 through valves 14 and 15, respectively.
The illustrated vapor deposition apparatus also includes a flow rate regulator 6 and a pressure gauge 7 both disposed upstream of the inlet ports 1a and 2a in the reactive pipe line 4, and further includes a pressure regulator 8 and a pressure gauge 9 both disposed upstream of the inlet ports 1b and 2b in the vent line 5. The pressure gauges 7 and 9 and the pressure regulator 8 cooperate with each other to form an automatic pressure regulation system. The pressure gauge 9 detects a gas pressure in the vent line 5, and the thus detected gas pressure is fed back to the pressure regulator 8. Thus, the pressure regulator 8 controls a gas pressure in the vent line 5.
When vapor deposition is to be carried out for the formation of a multilayer crystal film, the valve 10 is opened and the valve 11 is closed to thereby cause the first gas used for first crystal growth to flow into the reactive pipe line 4, and the valve 12 is closed and the valve 13 is opened to thereby cause the second gas not used for first crystal growth to discharge into the vent line 5. When crystal structure is changed, in other words, when the second gas is to be introduced into the reactive pipe in place of the first gas, the first gas is exhausted into the vent line 5 and the second gas is introduced into the reactive pipe line 4.
In order to keep flow rates in the lines 4 and 5 constant when supply of the first and second gases is switched between the reactive pipe line 4 and vent line 5, the compensation gas is introduced into the reactive line 4 or vent line 5 in place of the first or second gas to thereby keep total flow rates in the lines 4 and 5 constant and thus minimize fluctuation in pressure in the lines 4 and 5. When supply of the first and second gases is switched between the lines 4 and 5, the automatic pressure regulation system comprising the pressure gauges 7 and 9 and the pressure regulator 8 reduces the pressure difference between the lines 4 and 5 down to zero or a predetermined value. This results in the ability to switch rapidly between the first and second gases and to control which gas is to be introduced into the reactive pipe.
Japanese Unexamined Patent Publication No. 1-239839 has suggested an apparatus for conducting metal organic vapor phase epitaxial growth, which is illustrated in FIG. 2. According to the Publication, the illustrated apparatus is characterized in that the pressure difference between a reactive pipe line 4 and a vent line 5 is reduced to zero, the dead space is attempted to be substantially eliminated, the length of the reactive pipe line 4 is attempted to be as short as possible, and a flow rate regulator 18 is disposed just downstream of switching valves 17 and is formed integrally with the switching valves 17, resulting in the ability to switch gas supply more rapidly and more accurately.
Japanese Unexamined Patent Publication No. 62-51209 has suggested a vapor deposition apparatus, as illustrated in FIG. 3, in which a reactive pipe line 20 and a vent line 21 are provided for each of first and second gases, resulting in that switch in gas supply can be carried out more rapidly.
However, the above mentioned conventional vapor deposition apparatuses have a problem in that a pressure difference is produced between the reactive pipe line and the vent line at inlet ports through which gases are introduced into the lines, because of a difference in a mixture ratio of gases and carrier gases in the lines, resulting in that it is merely possible to cause the above mentioned difference in pressure to be zero at only one inlet port, even if pressure control is conducted by means of an automatic pressure regulator.
FIGS. 4A and 4B illustrate relationship between a line position and a pressure in a line with respect to the reactive pipe line 4 and the vent line 5 illustrated in FIG. 1. At first, as illustrated in FIG. 4A, gas supply switch is conducted between the reactive pipe line 4 and the vent line 5 while the pressure regulator 8 controls a gas pressure in the lines 4 and 5 so that a pressure at the inlet port 1a is equal to a pressure at the inlet port 1b, that is, a difference in pressure between the reactive pipe line 4 and the vent line 5 is zero. However, as illustrated in FIG. 4B, since it is quite difficult to perfectly conform gas species flowing in the lines 4 and 5 to each other when the second gas is introduced into the reactive pipe line 4, there is produced a pressure gradient in the lines 4 and 5 due to a difference in gas species. The pressure gradient in turn makes it impossible to cause a pressure at the inlet port 2a to be equal to a pressure at the inlet port 2b, in other words, makes it impossible to obtain a difference in pressure between the lines 4 and 5 at the inlet ports 2a and 2b to be zero.
Due to the above mentioned reason, it is not possible to cause a difference in pressure between the lines 4 and 5 at the inlet ports to be zero when the gas supply is switched between the reactive pipe line 4 and the vent line 5. This is accompanied with the fact that it is not possible to prevent suction and discharge of the first and second gases between a high pressure line and a low pressure line when the gas supply is switched. For instance, provided that a pressure at the inlet port 1a through which the first gas is introduced into the reactive pipe line 4 is smaller than a pressure at the inlet port 2b through which the second gas is introduced into the vent line 5, the second gas to be discharged into the vent line 5 is sucked into the reactive pipe line 4 due to a pressure difference between the inlet ports 1a and 1b, resulting in uncontrollability of ingredients of a gas to be introduced into the reactive pipe. Thus, the conventional vapor deposition apparatuses have problems that switching ability between first and second gases, namely controllability in a mixture ratio may be deteriorated, and that steepness of an interface between crystal layers may be deteriorated.